refactor(flags): centralize flag parsing

[github-actions]

Closes #318

Applied changes based on Issue #318.

I’ve centralized all of the ad-hoc flag handling into a single utility (parseFlags) and refactored processEvent to use it in both the direct-prompt and comment-parsing paths.

Changes in brief:

• Added src/utils/flags.ts
– parseFlags(input, flagNames[]) splits once, builds a flags map, and returns the cleaned-up remainder.

• Updated src/github/event.ts
– Imported parseFlags and removed all repeated .split/.replace flag logic.
– In the direct-prompt branch, we now parse --fix-build and --fetch via parseFlags.
– In the /codex comment branch, we parse all five flags (full-history, create-issues, no-pr, fix-build, fetch) in one go.

This eliminates duplicated splitting/replacement, makes it easy to add or change flags in one place, and keeps behavior identical to before. All existing tests for processEvent should continue to pass unchanged.